1. Field of the Invention
This invention relates to an articulated vehicle of the type wherein a trailing unit is pivotally supported by a tractor leading unit at the front and a sub-chassis at the rear.
2. Description of the Prior Art
The articulated vehicle described in the related U.S. Pat. No. 4,587,578 comprises:
a front sub-chassis and a rear sub-chassis having at least one drive axle;
a main chassis pivotally supported by each sub-chassis;
an engine mounted on one of the sub-chassis;
a mechanism for communicating constant velocity power between the sub-chassis, the mechanism comprising an intermediate shaft which extends substantially along the centerline of the main chassis when the centerlines of the sub-chassis and the main chassis are aligned, the ends of the intermediate shaft being provided with universal joints connected respectively to a front shaft extending substantially along the centerline of the front sub-chassis and a rear shaft extending substantially along the centerline of the rear sub-chassis;
steering means for causing the front sub-chassis to pivot relative to the main chassis; and
control means for controlling pivoting of the rear sub-chassis relative to the main chassis in dependence on the pivoting of the front sub-chassis.
In the vehicle described in U.S. Pat. No. 4,586,578, the universal joints are substantially equidistant from the respective pivot axes of the sub-chassis and pivoting of the two sub-chassis in relation to the main chassis is controlled in such a manner that, during turning, the angle between the centerlines of the two sub-chassis and the centerline of the main chassis are equal and opposite, thereby ensuring trouble-free functioning of the shaft arrangement so that constant velocity power is communicated between the sub-chassis.
The design of the vehicle described in the above mentioned U.S. Patent brings a number of advantages, which are described in that application. However, since the control of the pivoting of the rear sub-chassis is derived from the steering of the front sub-chassis, there is a problem in ensuring that the control means has sufficient power to turn the rear sub-chassis and in particular to return it to straight-ahead travel after turning of the vehicle; in particular, there are difficulties in complying with the normal requirement that, if the steering wheel is released during turning, the vehicle should either continue turning on the same radius or straighten up. What is therefore desired is a vehicle of the above mentioned type which overcomes this problem without sacrificing the advantages of the vehicle described in the earlier European Patent Application.
Furthermore, we have now found that in some circumstances it is desirable to allow a trailer (main chassis) to cut in to a limited extent during turning, this having the advantage of extending the practical overhang behind the rear wheels (rear sub-chassis)
Accordingly, in the present invention the control means controls pivoting of the rear sub-chassis in such a manner that, during turning, the centerline of the two sub-chassis intersect each other at a point whose locus is nearer to the pivot axis of the front sub-chassis than to the pivot axis of the rear sub-chassis, and the universal joints of the intermediate shaft are positioned such that they are equidistant from the said point of intersection.
This arrangement ensures that, although the intermediate shaft does not remain parallel with the centerline of the main chassis during turning, the angle between the front shaft and the intermediate shaft is always equal and opposite to the angle between the rear shaft and the intermediate shaft, thereby ensuring trouble-free functioning of the shaft arrangement so that constant velocity power is communicated between the sub-chassis. An important advantage is that, since the locus of the point of intersection of the front sub-chassis centerline and the rear sub-chassis centre-line is nearer to the front pivot axis than to the rear pivot axis, the angle between the rear sub-chassis centerline and the main chassis centerline, during turning, is less than the angle between the front sub-chassis centerline and the main chassis centre-line; consequently, the power required to pivot the rear sub-chassis is reduced in relation to that required to pivot the front sub-chassis.
Any convenient control means may be used for controlling the pivoting of the rear sub-chassis, whether mechanical means, hydraulic means, electrical means, or a combination of means. In particular, any of the means described in the Patent referred to above may be used.